The present invention relates to a device for protecting secondary batteries, and more particularly to a device for protecting secondary batteries charged with abnormal voltage.
Lithium ion batteries employing carbonaceous materials capable of doping or dedoping lithium ions are lightweight systems having high operating voltage and a high energy density, and so expected to have applications in the form of power supply secondary batteries for various types of portable equipment including mobile radio communications terminals such as portable phones, portable personal computers, and camcorders.
A secondary battery can be repeatedly used by charging. For charging, however, full-charging should be achieved for a short time of period while care is taken of prevention of overcharging that may otherwise result in a malfunction or breakdown of the battery.
A lithium ion battery is built up of an anode formed of a carbonaceous material capable of doping or dedoping lithium ions, a cathode formed of a lithium ion interlaminar compound such as at least one compound of Li.sub.x M.sub.y O.sub.z wherein M is Ni and/or Co and/or Mn, a separator formed of polyethylene, polypropylene or the like, and an electrolyte comprising a lithium salt dissolved in a non-aqueous organic solvent.
Upon overcharging of the lithium ion battery, the deposition of lithium on the anode occurs, and the internal pressure of the battery increases due to the generation of gas. When the internal pressure exceeds a certain threshold, an overpressure release valve is put into operation, resulting in an electrolyte leakage and, in the worse case, a breakdown of the battery.
According to one approach proposed to achieve safe full-charging of a lithium ion battery for a short time of period, the lithium ion battery is charged using a constant current at an initial charging stage and then using a constant voltage at the time when the charging voltage reaches a predetermined voltage.
Even when charging is carried out under controlled charging current and voltage conditions, however, there is a possibility that the voltage applied on the battery may increase by reason of anything abnormal. This voltage increase may in turn put the battery in an overcharged state.
In only a few applications, one single lithium ion battery is used. In other words, it is general that, in order to obtain predetermined voltage or current, a plurality of batteries are set in a battery room in equipment with which they are used, or a battery package having a plurality of batteries therein is detachably mounted in equipment with which they are used. Even in this case, there is still an undeniable possibility that trouble with one battery in service may have some adverse or grave influences on the remaining batteries.
To provide protection against overcharging, and overcurrents at the time of charging or in service as well as to prevent malfunctions of batteries for the reason of discharging at a voltage below the final discharge voltage of the batteries, various protective devices have been proposed. These protective devices are packed in a battery package or located in the vicinity of batteries.
For charging, a battery is connected to a charger to supply a current thereto. In some cases, the battery is placed in an overcharged state by the application of unacceptably high voltage thereto, which may be caused by a malfunction of the charger or the connection thereof to a charger operating at an improper voltage. This overcharged state may be eliminated as by a voltage-monitoring means in a battery-protecting device, which detects the voltage of the battery higher than a preset voltage to cut off a charging current. If the charger itself has a problem, however, repeated overcharging occurs until a voltage preset for the protective device is reached.
As a lithium ion battery is exposed to repeated overcharging, arborescent crystals of metal lithium grow on the surface of an anode formed of a carbonaceous material doping or dedoping lithium ions to such an extent that they break through a separator formed of polyethylene, polypropylene or the like. This in turn causes rapid reactions, resulting in a pressure increase in the battery. As the internal pressure exceeds a certain threshold, an overpressure release valve is actuated, ending up with electrolyte spills. In the worst case, there is a breakdown of the battery, thermal runaway, etc.
The present invention is concerned with a protective device for secondary batteries having a high energy density such as lithium ion batteries using a carbonaceous material doping or dedoping lithium ions as an active anode material. An object of the present invention is to provide a protective device which, when such batteries are repeatedly charged to an overcharged state, is actuated to disable them without detriment to their safety.